1. Field of the Invention
The invention concerns a method and system for bonding electrical devices using an electrically conductive adhesive.
2. Description of the Background Art
Traditionally, a wet conductive adhesive paste has been used to adhere a semiconductor die or chip to a substrate in die bonding. An external heating and pressurising means, which is referred to as a thermocompression means, is used to cure and press the adhesive paste underneath the die against the substrate so as to bond the die and the substrate together. In recent years, the continual reduction in thickness of dies from 500 to 25 μm has made a great impact on semiconductor die packages and their resulting electronic products as these thin dies not only enable ultrathin applications (e.g., smart cards, biological passports, etc.) but also promote ultrahigh-density applications (e.g., memories, CPUs, etc.). Problems of using a traditional adhesive paste in bonding a die of such thinness have been adhesive overflow and adhesive spread-out which cause an unavoidable short-circuit failure.
Dry conductive adhesive films or tapes such as Die Attach Film (DAF) or Thermoplastic Adhesive Tape (TAT) were developed to replace the traditional wet conductive adhesive pastes, especially in bonding thin dies of thicknesses less than 75 μm. DAF is usually laminated on the backside of the wafer before dicing the wafer into individual dies, while TAT is usually attached to the backside of individual dies after the wafer dicing process. A die laminated with DAF or TAT can be directly bonded to a substrate using heat and pressure which is referred to as thermocompression bonding and is also regarded as the state-of-the-art bonding method. Problems with this method have been a high bonding temperature of 100 to 180° C., a long bonding time of at least 3 seconds, a single die bonding per bonding cycle and a continual heating of fresh and post-bonded dies, adhesives and substrates throughout the bonding process. Yet void formation along the die-adhesive-substrate interfaces, a constrained bonding process window, a low throughput and the inability to use less expensive substrates such as synthetic resin bonded papers (SRBP, FR-1 and FR-2) which have lower operational temperatures than the bonding temperature of DAF or TAT have often occurred. The high temperature and lengthy heat exposure affects the reliability of the die package and bonding equipment.
WO 2007/061216 discloses a method for bonding electrical devices using flip chip ultrasonic vibration. As depicted in FIG. 2 of WO 2007/061216, the active (circuitry) surface of the die flips over or faces away from the ultrasonic transducer and connects to the substrate through the adhesive, while the inactive (ground) surface of the die makes contact with the transducer to ensure the active surface is not damaged by surface to surface friction from the transducer. To further mitigate the risk of damaging a die, the operational end of the transducer may be covered with a relatively hard, low damping, low friction and slippery Teflon cap. Heat is generated inside the adhesive itself as a by-product of the ultrasonic vibration. In other words, the ultrasonic vibration is applied to the inactive surface of the die which vibrates relative to the active surface of the die, the adhesive and the substrate, and this friction generates heat. WO 2007/061216 also discloses applying an additional amount of heat to all or some of the upper and lower bonding portions using an external heat source. WO 2007/061216 further discloses using a die collet for fixing the die at its inactive surface. The die collet also functions as a size adapter for the transducer when there is a size difference between the operational end of the transducer and the die. For example, there are die collets made from extra hard tungsten carbide with low internal damping.
The primary purpose of WO 2007/061216 is to ensure that ultrasonic vibration reaches the adhesive via the die. Therefore, the die must conduct the ultrasonic vibration to the adhesive. The by-product of the ultrasonic vibration reaching the adhesive is that heat is generated from the friction to the adhesive. However, the risk is that the die, especially thin dies, may be broken or powdering caused by the ultrasonic vibration transmitted through the die.
There is a desire for a bonding method and system to address at least some of the abovementioned problems.